1. Field of the Invention
The present invention relates to a velocity modulation microwave amplifier which is capable of selectively amplifying one of two or more microwave input signals in different frequency bands.
2. Description of the Related Art
Velocity modulation amplifier tubes which operate at microwave radio frequencies (RF) are widely used in communications, radar transmitters, and numerous other applications. The most common types of such amplifiers are klystrons and traveling wave tubes (TWTs). These amplifiers include an electron gun and focussing structure which generates a long cylindrical electron beam, an RF interaction structure which provides gain and power output by interaction with the beam, and a collector where the unused beam energy is converted to heat. The different types of amplifiers differ from each other principally in the configuration of the interaction circuit.
Klystron tubes include input and floating resonant cavities which cause velocity modulation and electron bunching of the beam, and one or more output cavities which extract RF energy by deceleration and demodulation of the bunched beam. Due to the relatively high quality factor (Q) of the resonant cavities, the bandwidth of a klystron tube tends to be relatively narrow.
In a TWT, the input RF energy propagates along a slowwave interaction structure in approximate synchronism with the electron beam. The bandwidth can be much larger than for a klystron, but the RF circuit is longer due to weaker interaction. To avoid regenerative oscillations arising from waves traveling both forward and backward in the structure, TWT circuits are severed into two or more independent sections. The increased length and complexity of a TWT makes this device generally more expensive than a klystron.
Hybrid velocity modulation tubes have also been developed which combine the features of uncoupled resonant cavity (klystron) and traveling wave structures. An extended interaction circuit (EIC) klystron uses long resonant cavities, each with several interaction gaps, in a configuration which resembles a traveling wave structure. Another hybrid structure combines a floating cavity klystron input section with an EIC output section. A detailed description of conventional velocity modulation amplifiers is found in a paper entitled "HIGH-POWER LINEAR-BEAM TUBES", by A. Staprans et al, Proceedings of the IEEE, vol. 61, no. 3, March 1973, pp. 299-330.
A conventional microwave amplifier, whether it be a klystron, TWT or hybrid, is capable of operating with usable efficiency only within a limited frequency band. In applications where operation in two or more widely separated frequency bands is required, it has generally been necessary to provide two separate microwave amplifier tubes, each with its own electron gun, collector, and power supply. This redundancy increases the size and cost of the system in which the amplifiers are employed.